This invention relates to lens molding and more particularly to injection molding apparatuses and techniques.
There have been several attempts, in the prior art, to mold both plus and minus polycarbonate ophthalmic lenses. Minus lenses are lenses that are thinner in their center and thicker along their edges, and plus lenses are lenses that are thicker in their center and thinner along their edges.
Methods and apparatuses for molding lenses have utilized both compression and injection molding techniques as well as a combination of the two. One problem frequently associated with injection molding of lenses is that the lenses produced often contain a knit line. The manner in which this knit line is formed is described below with reference to FIGS. 1a-1d.
Referring first to FIG. 1a, a cavity 29 of an injection molding apparatus is illustrated which includes an inlet 28. The injected molten plastic 35 is illustrated as the plastic first enters the cavity 29. Assume for sake of discussion that the lens being fabricated within cavity 29 has a thinner center 33, such center being thinner than the outer edge of the lens. As the plastic 35 enters the cavity 29 it tends to flow to the outer perimeter of the cavity 29, since the cavity is thicker in this region. In FIG. 1b the plastic 35 is again illustrated as it continues to fill the cavity 29. As may be seen in this figure, the plastic 35 does not immediately flow into the thinner center 33 of the cavity 29 but rather continues to advance about the outer rim of the cavity. In FIG. 1c the continued flow of the plastic 35 is illustrated, however, in this figure as may be seen, the plastic has flowed through the thinner center of the lens. As the plastic meets within the thinner center of the cavity a knit line 31 develops at the seam along which the flow meets. In FIG. 1d the cavity is illustrated completely filled with the plastic 35, and the knit line 31 which is inherent in conventional plastic flow (for any cavity having a thin center) is illustrated. This knit line prevents the use of the resultant blank as an optical lens without additional treatment, such as polishing.
Another problem associated with the formation of a lens in a single injection molding operation is that during the curing of the plastic, shrinkage occurs which results in an uneven and wavy exterior surface on the finished lens blank. Such shrinkage may also cause bubbles and other imperfections in the interior of the lens, and these defects will cause optical distortions and aberrations which are unacceptable for prescription lenses and instrument lenses.
Ideally, a lens should be produced in a single injection molding operation. As will be seen, the method and apparatus of the present invention provide for the injection molding of an optical lens which results in a finished lens blank which requires fewer finishing operations. The finished blank may be thin (approximately 1 millimeter) at its center so as to result in an optically correct, lightweight and asthetically pleasing eye glass lens which is shatter proof.
U.S. Pat. Nos. 4,088,031 and 4,091,057 disclose an apparatus and method for producing a lens in which a clear thermoplastic such as acrylic or polycarbonate is forced between the two mold halves which define the optical blank. These mold halves are forced away from each other as the cavity is filled with the molten plastic. An inner press which is disposed within the injection molding apparatus urges the mold halves together once the mold is filled to a certain level with the molten plastic. The urging of the mold halves together causes a certain amount of the molten plastic to be forced through an outlet port into a self-adjusting overflow pocket. This outlet port is spaced away from the inlet port, and there may be more than one outlet in communication with the cavity.
One problem associated with the method and apparatus utilizing transfer pockets is that additional finishing operations must be performed on the fabricated lens in order to remove the transfer pockets and the plastic that cured in the inlet port. In general, lenses would be produced at a much faster rate if these finishing operations could be reduced or eliminated.
It is therefore a principal object of the present invention to provide an apparatus and method which will produce lenses using a combination of injection and compression molding techniques in which molten plastic is injected into and compressed out of the mold through the same opening.
Another object of the invention is to provide a method and apparatus of producing an optical blank in which the lens finishing operations are kept to a minimum.